In orthodontic practice, forces are applied over time to malaligned teeth in order to gradually adjust their orientation as desired. Although numerous techniques are used for generating the necessary forces and applying them to the teeth, the most common system in use today includes an orthodontic archwire spanning the affected teeth, and cooperating orthodontic brackets mounted to the teeth which include one or more slots for receiving the archwire. The brackets typically include tie wings which are used to ligate an archwire into a receiving slot, generally by means of small elastic O-rings, other elastic ligatures, or orthodontic ligating wire.
To achieve ideal positioning of a malaligned tooth, it may be necessary to adjust the tooth elevation (up-down location within the socket), rotation (about the apical axis), tilt or tip (mesial or distal angular adjustment), and torque (lingually or facially). Where necessary, a combination of these adjustments may be made simultaneously by applying the appropriate forces to the tooth through the orthodontic bracket and, in some instances, additional appliances.
One of the simplest bracket types in widespread use today is the so-called "single" bracket, such as the Alexander "Mini Brackets" distributed by ClassOne.RTM. Orthodontics. Single brackets include a base for mounting to the tooth, typically by directly cementing the base to the enamel surface of the tooth. Alternatively, the base may be secured to a metal band which encircles the tooth in some applications. The single bracket includes a facially oriented slot for receiving the archwire, which is located in the center portion of present brackets. The slot may be dimensioned and angled to generate, in combination with the archwire, desired forces for transmission to the tooth. To secure the archwire in the slot, the single bracket typically includes a pair of opposed tie wings for securing an elastic or other type of ligature.
The size of a standard single bracket has been determined by a number of factors relating to its orthodontic performance, its necessary strength to resist forces acting against portions of the bracket, and the necessity for adhering the device securely to the tooth. With regard to orthodontic performance, in order to accomplish certain realignments (e.g. rotation) additional elements, such as contacting arms, are typically added which increase the physical size of the bracket. To allow the bracket to transmit the forces generated by these elements to the tooth, a substantial contact area and base size have been required, including a significant mesiodistal width. With regard to strength and attachment to the tooth, one of the principal factors determining the size of present brackets is the need to reliably resist the large external forces which periodically act against the bracket. Significant among these are the forces generated when biting and chewing, particularly the occlusal forces acting against brackets adhered to the incisors and bicuspids when biting through or chewing certain foods. These forces also tend to break the tie wings or tie wing supports, resulting in failure of the bracket.
To overcome these problems, single brackets typically have a robust size so that the tie wings and their supports are sufficiently strong to resist these forces. In turn, the base and contact area with the tooth must be relatively large to resist the dislodging occlusal forces generated by food acting against the substantial occlusal face of the bracket. Single brackets in present use are typically 3-3.5 millimeters in length on the apical axis, and 2.5 millimeters or more in mesiodistal width. The base areas of such brackets are typically 7 square millimeters and greater.
Another bracket type in widespread use is the so-called "twin" or "double" bracket, exemplified by the Alexander Mini Bracket twin distributed by ClassOne.RTM. Orthodontics, or the Unitek Miniature Twin bracket by 3M.RTM.. It includes two structures for cooperating with an archwire, each having a facially opening slot in the center portion and a pair of tie wings. Because the overall bracket is typically larger than a single bracket, sufficient base surface is provided to reliably mount the bracket against the occlusal dislodging forces encountered due to the substantial occlusal face of the device, and to transmit the desired alignment forces to the tooth.
Although techniques have been perfected which can accomplish the desired orthodontic realignments, there remain several shortcomings with the presently known systems. Of significant importance, the aesthetics of present systems continues to present a substantial impediment to more widespread acceptance. As noted, single and double brackets, even those promoted as "mini" or "miniature," are relatively large in comparison to the front surfaces of the teeth, particularly the narrow incisors and short crown bicuspids. Yet these are the teeth most noticeable when one smiles, making present brackets highly noticeable in use. Brackets have typically been manufactured from stainless steel or similar metals because of their substantial strength, ease of fabrication and relative biological inertness. Unfortunately, metals result in a bracket which is generally considered to be unsightly, and therefore there is substantial patient resistance to the present metal brackets.
Present brackets also result in undesirable adverse effects on the gingiva. Because of their substantial occlusal faces, they interfere with a substantial portion of the food passing over or near to the tooth surface when biting or chewing, preventing food from contacting portions of the gingiva. This deprives the affected gingiva of normal and desirable massaging, and can result in gingival deterioration. Large brackets also tend to collect and hold more food particles, which can exacerbate problems and lead to gingivitis. Further, although stainless steel is an excellent material for bracket construction, certain individuals exhibit allergic sensitivity to nickel, which is part of typical stainless alloys. These problems are exacerbated by the large quantity of stainless steel in close proximity to the gingiva required by present brackets, and may result in some cases in hypertrophic gingiva or fibrotic tissue growth.
In response to these problems, numerous orthodontic systems have been developed which include transparent (e.g. crystalline alumina), ceramic, or other tooth-colored or transparent materials. The intent of these brackets is to minimize the aesthetic impact of the bracket. Unfortunately, these alternative materials have mechanical disadvantages compared to stainless steel. Thus a non-metallic bracket having adequate physical strength to resist fracture must typically be correspondingly larger, and therefore more noticeable. This is particularly true where the bracket must accomplish a rotational realignment, and therefore must include additional structures to generate the desired rotation force. Brackets made from alternative materials also tend to be more expensive than metal brackets. Such unfortunate trade-offs continue to frustrate those interested in effective but aesthetically acceptable orthodontics.
Where additional rotation force is required in conjunction with existing brackets, it is known to utilize rotation wedges having an elastomeric body which is secured between the archwire and the tooth surface, next to a bracket. The force generated by the partially compressed body of the wedge supplements forces which may be generated by the bracket itself. One common type of rotation wedge includes a thin web having two apertures which may be passed over one of the pairs of tie wings on a double bracket. These are typically used only in connection with double brackets, since the web occupies one set of the wings and therefore requires another pair of tie wings to ligate an archwire. Alternatively, it is known to provide two relatively large ligating loops as part of the elastomeric wedge in combination with a particular bracket design, where the loops may be "crisscrossed" under and over the tie wings, to both secure the wedge in position and ligate an archwire. There are several shortcomings with these known devices. Those which require a separate pair of tie wings, and therefore use of a double bracket, result in an orthodontic appliance which is aesthetically displeasing, particularly for use on the incisors or bicuspids. The double ligations of the "crisscross" wedge also present an unsightly appearance. Further, the body portions of known devices have relatively large and flat occlusal surfaces which may generate substantial forces when contacted by food during biting or chewing. These forces are ultimately transmitted to the bracket, either directly or through the archwire, and can therefore tend to dislodge the bracket.
Accordingly, a need exists for an orthodontic system which can provide the necessary corrective forces for adjusting malaligned teeth in conjunction with an archwire, which is aesthetically acceptable to a larger number of users. In particular, there remains a need for an orthodontic bracket which minimizes the aesthetic problems associated with such devices. Simultaneously, there is a need for such a bracket which can resist fracturing or occlusal dislodging forces as part of a secure and affordable orthodontic system. There is also a need for a bracket and system which minimizes or eliminates adverse gingival effects, by permitting more normal massaging when biting, and by significantly reducing the volume of metallic alloys required. Where additional rotation forces are required, there is a further need for an orthodontic system including a rotation wedge which can simultaneously ligate an archwire to a single bracket, including a miniature bracket, while providing necessary rotational force and minimizing any additional dislodging forces acting on the bracket.